This invention relates to a method and apparatus for detecting and determining the amount of gas contamination of drilling mud and for measuring the amount of entrained gas in the drilling mud.
It has long been recognized that during drilling operations it is desirable to detect changes in the conditions in the formation at the lower portion of a drilling stem and in the borehole. In particular, it is well known in the art that it is necessary to maintain proper mud weight during drilling to obtain maximum drilling penetration and safety from blow-outs. Numerous attempts have been made to provide measurements of the amount of gas contamination of drilling fluids. In particular, U.S. Pat. No. 3,827,295 issued to the inventors herein on Aug. 6, 1974 discloses a device which gives continuous indications of changes in the condition at the bottom of the well. This device is useful in that it helps forewarn of difficulties or problems. Another type of device for determining the amount of changes in the drilling mud is shown in U.S. Pat. No. 3,911,741 issued to the same inventors herein on Oct. 14, 1975. This patent discloses a device for weighing the drilling mud after it has exited the bell nipple and discharge flow line and has been directed to the shaker box or the suction tank. U.S. Pat. Nos. 3,827,295 and 3,911,741 are incorporated herein in toto by this specific reference thereto.
As will be apparent, the above U.S. Pat. No. 3,911,741 discloses determining mud weight after it has exited the well. As will be apparent, entrained gases may have escaped, having been released to the atmosphere. Accordingly, it may not give an accurate measurement of what is actually going on in the well. In order to determine the weight of the mud in the mud column exiting from a well prior to the entrained gases being released, it is necessary that measurements be taken below the level of the mud at the top of the well, which is below the level of atmospheric pressure. The relationship of downhole containment pressures and reservoir pressures is the most important and dominant factor which affects the amount of gas which is mixed with the mud either from gas being flushed into the formation or gas which is drilled up with the gas bearing formation or, gas which is producing into the mud. It is therefore necessary to accurately determine the mud weight at the bottom of the well and the mud weight at the top of the well prior to escape of entrained gases. A comparison of these two weights provides a measure of the actual volume of gas in the mud. In this connection, Applicants have invented a method and apparatus for determining mud weight and gas content. It is an object of the invention to overcome problems found in the prior art and provide an accurate means for quantitatively analyzing the mud prior to exiting the bell nipple and before the escape of entrained gases. This provides an analysis based on the actual conditions of the mud based on what is actually going on in the well base and in the formations drilled.